Radio frequency energy sensitized compositions and method for sensitizing compositions to ratio frequency energy

ABSTRACT

A composition of enhanced radio frequency sensitivity and a method for providing enhanced radio frequency sensitivity to a composition in which a substance suitable for enhancement of radio frequency sensitivity is admixed with a radio frequency sensitizing amount of an inorganic radio frequency sensitizer treated essentially to remove absorbed water and/or other volatiles with the radio frequency sensitizer chosen from among zinc oxide (American Process), bentonite clay, and crystalline or amorphous alkali or alkaline earth metal aluminosilicate. A process for molding using these compositions and the molded product. The composition, method for enhancing radio frequency sensitivity, process for molding and molded product with a polymeric substance, particularly ultra high molecular weight polyethylene, as the substance suitable for enhancement of radio frequency sensitivity.

This application is a continuation application of applicant Ser. No. 008,814 filed Jan. 30, 1987 now U.S. Pat. No. 4,790,965, which is a divisional application of Ser. No. 707,613 filed Mar. 4, 1985, now U.S. Pat. No. 4,661,299.

BACKGROUND OF THE INVENTION

This invention relates to the enhancement of sensitivity of compositions to radio frequency energy. In one of its aspects it relates to compositions that are transparent to radio frequency energy. In another of its aspects this invention relates to compositions that have low sensitivity to radio frequency energy. In another of its aspects this invention relates to compositions whose sensitivity to radio frequency energy has been enhanced.

It is known that sold hygroscopic fillers can be added to certain polymer compositions to provide receptivity to microwave (MW) radiation (See U.S. Pat. No. 4,234,636). It has been noted that the microwave receptivity of these polymer compositions is significantly reduced by drying the fillers before compounding with the polymers. It has, therefore, been suggested that water associated with the hygroscopic filler is responsible for the MW receptivity.

It is also known that polar organic compounds can be admixed with particulate inorganic materials such as silicas to provide MW sensitizing compositions suitable for blending with polymers to provide MW receptivity to the resulting polymer compositions (See U.S. Pat. No. 4,360,607).

It has now been found that certain inorganic compounds can provide radio frequency (RF) radiation receptivity to a wide variety of compositions, especially polymers after these inorganic compounds have been essentially dried and in the absence of added polar organic compounds. This invention, therefore, provides many opportunities to utilize RF radiation for heating of compositions containing these certain inorganic compounds so that a multiplicity of products, such as molded objects, can be produced for a multiplicity of purposes.

It is, therefore, an object of this invention to provide a method for enhancing the radio frequency sensitivity of compounds to which the process is applicable. It is another object of this invention to provide compositions of enhanced radio frequency sensitivity. It is still another object of this invention to provide compositions containing certain inorganic compounds so that these compositions have enhanced radio frequency sensitivity as compared to the compositions without these inorganic compounds. It is still another object of this invention to produce molded objects by subjecting compositions of enhanced radio frequency sensitivity to radio frequency energy in a molding operation such as transfer, injection or compression molding. It is still another object of this invention to enhance the radio frequency sensitivity of suitable polymeric materials, particularly ultra high molecular weight polyethylene.

Other aspects, objects and the various advantages of this invention will become apparent upon reading this specification and the appended claims.

STATEMENT OF THE INVENTION

In accordance with this invention there are provided compositions in which there is a substance suitable for enhancement of radio frequency sensitivity and a radio frequency sensitizing amount of an inorganic radio frequency sensitizer treated essentially to remove absorbed water and/or other volatiles said sensitizer chosen from the group consisting of (1) zinc oxide (American Process), (2) bentonite clay, and (3) crystalline or amorphous alkali or alkaline earth metal aluminosilicate.

In an embodiment of the invention a method is provided for enhancing the radio frequency sensitivity of a substance suitable for such enhancement in which there is admixed with the substance suitable for radio frequency enhancement a radio frequency sensitizing amount of an inorganic radio frequency sensitizer treated essentially to remove absorbed water and/or other volatiles with a sensitizer chosen from the group consisting of (1) zinc oxide (American Process), (2) bentonite clay, and (3) crystalline or amorphous alkali or alkaline earth metal aluminosilicate.

In another embodiment of the invention compositions in accordance with the radio frequency enhancement of this invention are subjected to radio frequency energy in a process for producing molded objects.

The suitable inorganic RF sensitizers employed in the instant invention are selected from the group consisting of: zinc oxide (American Process), bentonite clay, and crystalline or amorphous alkali or alkaline earth metal aluminosilicates.

"American process" zinc oxide is a well known commercially available material. As distinguished from the "French process" material, the "American process" zinc oxide is obtained by roasting of a suitable zinc ore whereas the "French process" employs an oxidation of vaporized zinc metal. An example of a suitable zinc oxide is AZO-55 from American Smelting and Refining Co. Columbus, Ohio with typical properties as shown below:

    ______________________________________                                         TECHNICAL VALUES (Average)                                                     ______________________________________                                         Physical Properties                                                            Specific Gravity     5.6                                                       One Pound Bulks Gallons                                                                             0.0214                                                    Apparent Density (Lbs./cu. ft.)                                                                     32                                                        Particle Diameter (Microns)                                                                         0.27                                                      Surface Area (Sq.m./gm.)                                                                            4.00                                                      Rub-out Oil Absorption                                                                              14                                                        Fineness thru 325 Mesh                                                                              99.95                                                     (%)                                                                            Chemical Analysis                                                              Zinc Oxide (ZnO)     99.20%                                                    Lead (Pb)            0.02%                                                     Cadmium (Cd)         0.05%                                                     Sulfur (S)           0.03%                                                     H.sub.2 O Soluble Salts                                                                             0.30%                                                     Insoluble in HCl     0.15%                                                     Loss at 105° C.                                                                              0.10%                                                     ______________________________________                                    

Bentonite clay, particularly the "Western type", is disclosed in U.S. Pat. No. 4,234,636 as a hygroscopic filler in the prior art process noted. However, bentonite clay, especially the Western type, is also suitable in the instant invention after a heating step to drive off adsorbed water. Bentonite clay is a naturally occurring material containing appreciable amounts of the clay mineral montmorillonite, (Mg, Ca) O·Al₂ O₃ ·5SiO₂ ·nH₂ O. As noted above bentonite clay obtained from the Western states of the USA is preferred for use in the instant invention. A particularly useful Western bentonite material is Nygel-5 from Industrial Minerals Ventures, Inc. Lathrop Wells, Nevada.

The alkali or alkaline earth metal aluminosilicates are a preferred group of RF sensitizers according to this invention. These materials can be either amorphous or crystalline in nature and can be obtained by synthetic methods. The alkali metal ion can be lithium, sodium, or potassium preferably sodium, while the alkaline earth metal ion can be magnesium or calcium, usually calcium. It is also possible for combinations of the above ions to be present in the aluminosilicates such as sodium and calcium. Suitable amorphous aluminosilicates are exemplified by the commercially available Zeolex 23, Zeolex 7-UD, or Zeolex 7A from J. M. Huber Corp. Havre de Grace, Md. The suitable crystalline aluminosilicates encompass the well known crystalline zeolites which are crystalline compounds composed of tetrahedrons of silicon oxide in which aluminum atoms have replaced some of the silicon. The zeolites are well known for their ion exchange behavior and molecular sieve character. Suitable synthetic crystalline zeolites include those within the families of Type A, Type D, Type L, Type R, Type S, Type T, Type X, Type Y and Type ZSM-5. More specific examples include Types 3A, 4A, 5A, 10X, 13X, ZSM-8 and ZSM-11.

The average particle size (diameter) of the inorganic RF sensitizers of this invention is generally within the range of about 0.25 micron to about 15 microns, preferably from about 0.5 micron to about 7 microns. The surface area (BET method) can range from about 1 m² /g to about 1,000 m² /g, preferably from about 200 m² /g to about 500 m² /g. For best results, if a material with low surface area is chosen the particle size should be as small as practicable. Conversely, if a material of large particle size is chosen the surface area should be as high as practicable. In some instances it may be desirable to subject the material to a preliminary size reduction step to improve its effectiveness as an RF sensitizer in the application under consideration. This also may be especially helpful when the RF sensitizers of this invention are intended for use in polymer compositions. It is generally true that the physical properties of polymer are least affected when the particle size of the added material is at the smallest end of the range. Size reduction can be accomplished by conventional methods such as grinding, ball milling and the like.

As noted above the inorganic RF sensitizers of the instant invention are subjected to a preliminary treating step which effectively removes adsorbed water and/or other volatiles which might be present. The conditions employed in the treating step can vary widely depending on the amount of adsorbed water and/or other volatiles present among other considerations. Usually heating to a constant weight will be adequate to provide an inorganic RF sensitizer of this invention free of adsorbed water and/or other volatiles. Generally, the temperature can range from about 100° C. to about 300° C. while the time can range from about 0.5 hour to about 72 hours in the heating step of this invention. The heating can be carried out at atmospheric pressure and in a flowing gas such as air, nitrogen, argon or helium if desired. The heating also can be conducted under reduced pressure such as achieved by conventional vacuum pumps.

The inorganic RF sensitizers of this invention which have been treated as described above can be mixed with the material to be subjected to RF radiation in a variety of conventional methods.

For polymer compositions it is particularly effective and preferred to dry blend the inorganic RF sensitizer with solid polymer particles which may contain other conventional additives such as anti. oxidants, pigments, fillers and the like. The polymer particles will preferably have an average particle size of from about 10 mesh to about 40 mesh though smaller or larger particles can be employed if desired. It is also possible to use a melt blending operation to incorporate the inorganic RF sensitizer into a polymer composition such as by employing a mixing extruder. The RF sensitizer can be added before, after, or during addition of other polymer additives in the mixing extruder operation.

It is also possible to prepare a dispersion of a finely divided inorganic RF sensitizer in a liquid which is essentially inert to RF radiation e.g. alkanes having 5-30 carbon atoms, and then contacting the material to be made RF sensitive with said dispersion as by spraying, milling, tumbling or stirring without or with subsequent removal of the dispersion vehicle such as by vacuum assisted evaporation.

For RF transparent polymers the amount of inorganic RF sensitizer employed according to this invention can be broadly from about 1 wt. % to about 20 wt. % based on the weight of the total composition. Preferably, the amount will range from about 4 wt. % to about 6 wt. % on the same basis as above. These ranges are also particularly suited to RF equipment of about 1 kW power at 100 MHz frequency. For RF equipment of higher power and frequency it would be expected that the amounts of RF sensitizer can be reduced.

Furthermore, if the polymer employed has some RF responsiveness then the amount of RF sensitizer can also be reduced since the RF absorption effects of polymer and sensitizer are additive.

In the USA the Federal Communications Commission (FCC) has designated certain radio frequency bands centered about 50 MHz, 915 MHz, 2450 MHz, 5800 MHz and 22000 MHz for industrial heating units to minimize conflict with communication systems. The compositions which are RF sensitized according to the instant invention can be treated with electromagnetic radiation of preferably from about 1 MHz to about 2500 MHz, more preferably from about 20 MHz to about 1000 MHz.

Several manufacturers provide equipment suitable for generating the RF radiation within the needed ranges. An example is Thermall #9 Electronic Heating Equipment with 1.25 kW power at 100 MHz made by W. T. LaRose & Associates Troy, N.Y.

Power absorption is governed by the equation P=1.41 (E/D)² ×f×E" where P is in watt/in³, E is the electrode voltage in kV, D is the electrode distance in inches, f is frequency in MHz and E" is the dielectric loss factor which is the product of the dielectric constant, E', and dissipation factor, tan Δ. This equation is well known to those skilled in the art of RF or MW heating and can be used to adjust available equipment to the compositions being treated with RF energy.

Broadly speaking, the inorganic RF sensitizers of the instant invention are useful in essentially any composition wherein the inorganic compound is not materially detrimental and which is to be subjected to RF radiation for heating purposes. Thus, the inorganic RF sensitizers of this invention provide opportunities to use RF radiation for heating in a multitude of applications.

One area of application that is especially important involves polymer processing. In operations such as injection molding, transfer molding, blow molding, vacuum forming, extrusion, softening, foaming, shaping, curing and the like the RF sensitized polymer compositions can be subjected to RF radiation to efficiently provide the needed heat to accomplish the desired operation. The inorganic RF sensitizers can be used in presence of other polymer compounding ingredients such as carbon black which are also RF sensitive to some extent to enhance the level of RF sensitivity of the composition. This enhancement can also be obtained with the use of the inventive inorganic RF sensitizers in certain polar polymers which are in themselves RF sensitive. One particular area within the field of polymer processing that is especially aided by the inorganic RF sensitizers of this invention is that of handling ultra high molecular weight polyethylene (UHMWPE). it is recognized that this material has many desirable properties but the ultra high molecular weight which is apparently responsible for the desirable properties also makes this material extremely difficult to process. RF heating has been utilized with carbon black as the RF sensitizer but this has not been entirely satisfactory especially if articles having color other than black are desired. According to the instant invention the UHMWPE material can be intensively mixed with an inorganic RF sensitizer of this invention and then subjected to RF radiation during the molding, e.g. transfer, injection, or compression molding, of various articles therefrom.

The following are processes and compositions suitable in processes illustrative of the present invention. These examples should be taken as illustrative and should not be considered to be restrictive.

EXAMPLE I

A large number of inorganic powders was tested for RF responsiveness before and after drying (heating) at 138° C. (280° F.) for 18 hours. The tests were conducted by placing a weighed portion of the inorganic powder in a Teflon dish sample holder of 3 3/32 inch cavity diameter and 1/2 inch deep which could be covered with a 1/4 inch thick Teflon lid. The sample holder was placed between 6 inch×6 inch electrode platens set 11/2 inches apart in a Thermall #9, 1.25 kW, 100 MHz heater made by W. T. LaRose and Associates Troy, N.Y. Each sample was exposed to the RF radiation for the indicated time while the current flow in milliamperes (mA) and temperature reached (°F.) by the sample were noted. The background or "no-load" current reading was 167 mA. Temperature measurement was obtained with a needle pyrometer inserted into the sample at several points and the highest reading recorded. Temperatures greater than about 600° F. were estimated. Table Ia presents results obtained for a number of inorganic powders which after drying were rated as "nonresponsive" to "fair" response. Table Ib presents results for certain amine treated or silane treated clays which were rated as having "moderate" response after drying. It should be noted that the clay treating agents such as amino compounds would be expected to add polarity to the clays and thus provide some degree of RF sensitivity. The materials in Tables Ia and Ib are thus outside the scope of the instant invention. Table Ic presents results on inorganic powders which after drying show "Good" to "Excellent" RF response and these illustrate the RF sensitizers of the instant invention

It is surprising that the American process zinc oxide was "very good" in RF response (Table Ic) after drying while the French process zinc oxide is essentially "nonresponsive" even before drying. In addition, it is surprising that the other materials in Table Ic were "good" to "excellent" in RF response after drying.

                                      TABLE Ia                                     __________________________________________________________________________     Example I                                                                      Results of RF-Response of Various Inorganic Powders                            None - Fair Response                                                                               Amount                                                                              Exposure,                                                                            Not Dried                                                                              Dried                                   Material            Grams                                                                               Seconds                                                                              mA   °F.                                                                        mA  °F.                                                                        Observations                     __________________________________________________________________________     Precipitated Hydrated Silicas                                                  Hi Sil 233 (PPG)    25   30    50-30                                                                               250                                                                               2-7 185                                                                               Condens.sup.a both cases         Silene D (PPG)      15   30    35-46                                                                               275                                                                                5-10                                                                              231                                                                               Condens.sup.a both cases         Sipernat 22 (Degussa)                                                                              17   30    35-34                                                                               262                                                                                2-12                                                                              215                                                                               Condens.sup.a both cases         Other Silicas                                                                  Syloid 74 Silica gel                                                                               25   30    15-12                                                                               165                                                                                8-11                                                                              196                                 (Davison)                                                                      Syloid 308, hydrophobized                                                                          25   30    20-15                                                                               183                                                                               0-6 148                                 silica gel                                                                     Imsil A-108 (Ind. Min. Sil)                                                                        25   30    10-13                                                                               147                                                                               3-7 130                                 Carbonates                                                                     Altowhite UF, CaCO.sub.3                                                                           25   30    0-6  105                                                                               0-3 130                                 (Georgia Marble)                                                               Magcarb L, MgCO.sub.3 (C. P. Hall)                                                                 12.5 30    145-65                                                                              360                                                                               25-47                                                                              380                                                                               Fair Response                    ZnCO.sub.3          25   30    180-45                                                                              345                                                                               0-6 180                                 Talc, Mica                                                                     Emtal 549 (C. P. Hall)                                                                             25   30    2-8  115                                                                               0-3 155                                 Mica P-12 (C. P. Hall)                                                                             25   30    40-32                                                                               280                                                                               11-22                                                                              150                                 Calcined Clays                                                                 Polyfil 70 (J. M. Huber)                                                                           25   30    0-7  120                                                                               0-4 160                                 Hydrated Clays                                                                 B-80 (Thiele)       25   30    200-47                                                                              420                                                                               30-75                                                                              425                                                                               Condens.sup.a both cases         Barden-R (J. M. Huber)                                                                             25   30    170-42                                                                              405                                                                               45-35                                                                              230                                                                               Condens.sup.a undried            Champion Clay (Harwick)                                                                            25   30    160-37                                                                              440                                                                               16-26                                                                              215                                                                               Condens.sup.a undried            Hydrite UF (Georgia Kaolin)                                                                        25   30    102-39                                                                              390                                                                               0-6 160                                                                               Condens.sup.a undried            Oxides, Etc.                                                                   Aluminum oxide P (Degussa)                                                                         7.5  30    235-37                                                                              240                                                                               20-25                                                                              280                                                                               Condens.sup.a undried                                                          Fair Response                    Hydrated alumina    25   30    10-18                                                                               130                                                                                6-11                                                                              122                                 MAF Magnesium Oxide 25   30     4-11                                                                               110                                                                               3-8 85                                  Titanium oxide P-25 (Degussa)                                                                      10   30    30-25                                                                               200                                                                                3-10                                                                              175                                 Titanium oxide OR-450                                                                              25   30     8-15                                                                               145                                                                               3-7 205                                 (DuPont)                                                                       French Proc. ZnO Pasco 558                                                                         27   30    10-15                                                                               115                                                                               --  -- Not tested dried                 Lithopone           25   30    10-18                                                                               130                                                                                6-11                                                                              122                                 Hydrated lime       25   30    50-35                                                                               285                                                                                5-10                                                                              120                                                                               Condens.sup.a undried            Magnesium hydroxide 25   30    55-40                                                                               322                                                                               --  -- Decomposed when                                                                dried                            Diatomaceous Earth, Fullers Earth, Etc.                                        Dicalite 103 (Gretco)                                                                              12.5 30    105-50                                                                              468                                                                               12-17                                                                              225                                                                               Condens.sup.a undried            Celite Silver Frost 12.5 30    50-40                                                                               325                                                                               14-17                                                                              175                                                                               Condens.sup.a undried            (Johns-Manville)                                                               Fullers Earth       25   30    165-73                                                                              500                                                                                5-10                                                                              160                                                                               Condens.sup.a undried            Felex 100, Feldspar 25   30    10-14                                                                               150                                                                               1-7 142                                 (Feldspar)                                                                     __________________________________________________________________________

                                      TABLE Ib                                     __________________________________________________________________________     Example I                                                                      Results of RF-Response of Various Inorganic Powders                            Moderate Response                                                              Material       Amount                                                                              Exposure,                                                                            Not Dried                                                                              Dried                                        Treated Clays  Grams                                                                               Seconds                                                                              mA   °F.                                                                        mA   °F.                                                                        Observations                                                                            RF-Resp.                    __________________________________________________________________________     Nucap 190, Mercaptosilane                                                                     25   30    305-60                                                                              560                                                                               20-62                                                                               375                                                                               Condens.sup.a                                                                           Mod. -d                     (J. M. Huber)                                                                  Nulok 390, Aminosilane                                                                        25   30    200-62                                                                              550                                                                               20-42                                                                               330                                                                               Condens.sup.a                                                                           Mod. -d                     (J. M. Huber)                                                                  Imvitone II Amine Bent. Clay                                                                  25   30     135-285                                                                            480                                                                                5-205                                                                              480                                                                               Condens.sup.a                                                                           Mod. +d                     (Ind. Min. Vent.)         5 sec.          Smoke evolution,                                                               charring                             __________________________________________________________________________

                                      TABLE Ic                                     __________________________________________________________________________     Example I                                                                      Results of RF-Response of Inorganic Powders                                    Good-Excellent Response                                                                           Amount                                                                              Exposure,                                                                            Not Dried                                                                               Dried                                   Material           Grams                                                                               Seconds                                                                              mA   °F.                                                                         mA   °F.                                                                         Observations                                                                             RF-Resp.             __________________________________________________________________________     Oxides                                                                         Am. Proc. Zinc oxide, B                                                                           25   10    50-41                                                                               305 50-41                                                                               1100                                                                               GLOW.sup.b when                                                                          V. Good              (Smith C & C)                               (Est.)                             Amorphous Sodium Aluminum Silicates                                            Zeolex 7UD (J. M. Huber)                                                                          12.5 10    215-205                                                                             1100                                                                               20-195                                                                              1100                                                                               GLOW.sup.b in                                                                            Exc.                                                    Est.     Est.                                                                               cases                          Zeolex 23 (J. M. Huber)                                                                           10   10    175--175                                                                            1100                                                                               25-180                                                                              1100                                                                               GLOW.sup.b in                                                                            Exc.                                                    Est.     Est.                                                                               cases                          Zeolite Crystalline Sodium                                                     Aluminum Silicates                                                             Zeolite 4A, Detergent grade                                                                       12.5 35    265-255                                                                             1100                                                                               135-215                                                                             1100                                                                               Condens.sup.a in                                                                         Exc.                 (Union Carb.)                      Est.                                                                               10 sec.                                                                             Est.                                                                               cases. GLOW.sup.b in                                                           both cases                     Bentonite Clays                                                                Nygel (Ind. Min. Ventures)                                                                        25   30    130-195                                                                             800 70-167                                                                              1100                                                                               Condens.sup.a                                                                            V. Good                                                 Est.     Est.                                                                               GLOW.sup.b when dried          Rheospan (Am. Colloid)                                                                            25   30    220-120                                                                             700 45-101                                                                               900                                                                               Condens.sup.a                                                                            Goodied                                                 Est.     Est.                               HPM-20 (Am. Colloid)                                                                              25   30    150-120                                                                             575 65-110                                                                              1000                                                                               Slight condens.sup.a                                                                     Good                                                             Est.                                                                               undried. Condens.sup.a                                                         undried.                       325 Mesh (H. M. Royal)                                                                            25   30    85-98                                                                               390 60-80                                                                                800                                                                               Flames,                                                                                  Goodring                                                         Est.                               __________________________________________________________________________      .sup.a Condensate observed on RF exposure.                                     .sup.b Material glowed on RF exposure.                                   

EXAMPLE II

Other tests were conducted with the same apparatus employed in Example I in which amorphous sodium aluminum silicates (Table Ic, Example I) were admixed with several RF transparent polymers to provide polymer compositions with RF sensitivity. The results shown in Table II below clearly demonstrate the effectiveness of the dried amorphous sodium aluminum silicates in providing RF sensitivity to otherwise RF transparent polymers.

                                      TABLE II                                     __________________________________________________________________________     Example II                                                                     Run     Wt. %                                                                              Sample                                                                             Exposure,                                                                            Current,                                                                            Temp.                                               No.                                                                               Polymer                                                                             Zeolex                                                                             wt, g                                                                              sec   mA   °F.                                                                          Observation                                    __________________________________________________________________________     1  A.sup.(a)                                                                           0   30  60    0-5   85  --                                             2  A    .sup. 5.sup.(e)                                                                    30  60    55-58                                                                               500  melted                                         3  B.sup.(b)                                                                           0   12.5                                                                               30    0-5   85  --                                             4  B      8.sup.(d)                                                                        12.5                                                                               30    35-40                                                                               580  melted                                         5  C.sup.(c)                                                                           0   25  30    0-4   85  --                                             6  C    .sup. 10.sup.(d)                                                                   25  60    35-38                                                                                600+                                                                               melted                                         __________________________________________________________________________      .sup.(a) Polystyrene pellets.                                                  .sup.(b) Poly(phenylene sulfide) (PPS), Ryton ® powder type from           Phillips Petroleum Co. having a melt flow of 850 ± 100 g/10 min as          determined according to ASTM D 1238, Procedure B modified to use 5 kg          weight 600° F. (316° C.) and 0.17 mm orifice.                    .sup.(c) Poly(tetrafluoroethylene), Teflon ® powder from I. E. duPont      Co. ground to a powder.                                                        .sup.(d) Zeolex 23 from J. M. Huber Corp.                                      .sup.(e) Zeolex 7 from J. M. Huber Corp.                                 

EXAMPLE III

Tests were conducted which examined the effect of added amorphous sodium aluminum silicate on the microwave curing of a rubber composition containing carbon black. The compounding recipe is shown below.

    ______________________________________                                         Recipe                                                                                           Parts, by wt                                                 ______________________________________                                         Rubber.sup.(a)      100                                                        Oil and stabilizers 11.5                                                       Sulfur              0.5                                                        Zinc dimethyldithiocarbamate                                                                       3                                                          Zinc dibutyldithiocarbamate                                                                        3                                                          Carbon black, N-330 65                                                         Carbon black, N-550 30                                                         Calcium oxide in oil                                                                               10                                                         4,4'-Dithiodimorpholine                                                                            2                                                          Sodium Aluminum silicate.sup.(b)                                                                   0 or 9                                                     ______________________________________                                          .sup.(a) Oil extended (50 phr oil) ethylene/propylene/diene monomer (EPDM      rubber.                                                                        .sup.(b) Zeolex 23 from J. M. Huber Corp.                                

The compounds were mixed then fed in a continuous manner to a vacuum extruder and the extrudate fed to a microwave curing oven (10 KW Despatch oven made by Despatch Oven Co.) with surrounding air kept at 500° F. to prevent heat loss then to a hot air oven with 500° F. hot air then through a water bath for cooling before take off on a turntable. At an extrudate cross section of about 0.4 square inch the results shown below were obtained.

    ______________________________________                                                           Run IIIa                                                                              Run IIIb                                                                (Control)                                                                             (Invention)                                           ______________________________________                                         Zeolex 23, phr      0        9                                                 Linear speed, ft/min                                                                               20       27                                                Microwave power setting, KW                                                                        8        4                                                 ______________________________________                                    

At an extrudate cross section of about 0.1 square inch the invention compound gave blistered product which indicated excessive temperature during curing which in turn indicated that 9 phr of the amorphous sodium aluminum silicate was too much for the conditions used. However, the results shown above for the thicker extrudate demonstrate a strong microwave cure promoting effect for the Zeolex 23 additive. This is seen by increased oven throughput and a reduced microwave power output requirement. It should also be noted that the cure promoting effect was evident even though large amounts of carbon black were present in the compound. Carbon blacks are known to promote microwave responsiveness in rubber compositions.

EXAMPLE IV

The RF responsiveness of both amorphous and crystalline sodium aluminum silicates according to this invention was utilized to prepare sintered polymer compositions useful as filters and the like. These compositions are especially useful when made from ultrahigh molecular weight polyethylene (UHMWPE) because the resulting sintered polymer compositions have excellent strength properties even though they are porous.

In these tests the polymer powder (Hercules UHMWPE 1900 IV 22 or 27) and RF sensitizing additive were ballmilled for 45 minutes in a glass jar with steel balls. The resulting mixture (about 18 g) was placed in the Teflon sample holder of the RF heating apparatus described in Example I which served as the mold. In these runs the electrode platen distance was 15/8 inch. Each mixture was subjected to very light mold pressure of 5.5 g/cm² (none in Run 7 of Table IVa) during the RF radiation treatment. The results obtained in these runs are presented in Table IVa and IVb below. The runs in Table IVa were made with an amorphous sodium aluminum silicate, Zeolex 7 UD from J. M. Huber Corp as the RF sensitizer while the runs in Table IVb were made with a crystalline sodium aluminum silicate, Arogen 3001 also from J. M. Huber Corp. as the RF sensitizer. Except as indicated for Runs 8 and 11-13 of Table IVb the mold was exposed to RF radiation at ambient temperature, i.e. no mold preheating.

                                      TABLE IVa                                    __________________________________________________________________________     Hercules UHMWPE 1900 IV 22                                                     Run                                                                               Sensitizer,                                                                          Exposure                                                                            Max-Min                                                                             Calculated                                                  No.                                                                               php   time, sec                                                                           mA   % Porosity.sup.(b)                                                                    Observations                                         __________________________________________________________________________     1  5     240  35--35                                                                              --     Much loose powder                                    2  5     330  40-35                                                                               --     Some loose powder,                                                             marginal.                                            3  10    120  60-40                                                                               49     Good, but some loose                                                           powder on surface.                                   4  15    90   68-65                                                                               41     Very good. No loose                                                            powder.                                              5  20    90   70-65                                                                               42     Very good. No loose                                                            powder.                                              6  25    70   100-90                                                                              48     Fair. Loose powder                                                             on surface.                                          7.sup.(a)                                                                         20    90   68-62                                                                               43     Fair. Some loose                                                               powder on surface.                                   __________________________________________________________________________      .sup.(a) No pressure applied to mold.                                          .sup.(b) Ratio of product specific gravity over polymer specific gravity       × 100.                                                             

                                      TABLE IVb                                    __________________________________________________________________________     Hercules UHMWPE 1900 IV 22.sup.(a)                                             Run                                                                               Sensitizer,                                                                          Exposure                                                                            Max-Min                                                                             Calculated                                                  No.                                                                               php   time, sec                                                                           mA   Porosity.sup.(b)                                                                      Observations                                         __________________________________________________________________________     1   5    180  42--42                                                                              49     Good, but some loose                                                           powder on surface.                                   2   5    240  43--43                                                                              47     Excellent                                            3  10    120  90-70                                                                               50     Excellent                                            4  15    90   112-105                                                                             52     Excellent                                            5  20    60   150-140                                                                             50     Very good                                            6  25    20   200- 50     Poor, much loose                                                               powder on surface.                                   7  10.sup.(b)                                                                           90   96-47                                                                               51     Excellent                                            8.sup.(c)                                                                         10.sup.(b)                                                                           60   100-50                                                                              54     Excellent                                            9.sup.(a)                                                                         10    120  90-72                                                                               51     Excellent                                            10.sup.(a)                                                                        10.sup.(d)                                                                           120  90-70                                                                               --     Poor, not sintered.                                  11.sup.(c)                                                                        15    60   135-125                                                                             50     Excellent                                            12.sup.(c)                                                                        15    30   135-125                                                                             49     Very good                                            13.sup.(c)                                                                        15.sup.(e)                                                                           45   137-72                                                                              51     Excellent                                            __________________________________________________________________________      .sup.(a) Runs 9 and 10 employed IV 27 type polymer.                            .sup.(b) Added 1 php of Nethyl-o and ptoluene-sulfonamide.                      .sup.(c) Mold temperature increased to 320° F. before RF exposure      .sup.(d) Added 1 php of glycerine.                                             .sup.(e) Added 2 php of an azodicarbonamide blowing agent (Porofor ADC/M       from Mobay Chemicals) and 0.5 php of American process type zinc oxide.   

The results in Tables IVa and IVb above show that sintered polymer compositions of suitable porosity and strength for use as filters can be readily made from UHMPE and amorphous or crystalline sodium aluminum silicates employing RF heating. It was further shown that such filter materials have a very high absorption capacity for ordinary tobacco smoke in that 20 mouthfuls of cigarette smoke blown through the same filter area left only a small depth discolored.

It can also be seen that at about 25 php of inorganic RF sensitizer there is interference with sintering of the UHMWPE polymer particles. In these tests about 10-15 php of sensitizer seemed to give best results.

Comparison of Runs 7 and 8 and Runs 4 and 11 of Table IVb show that preheating the mold to about 300°-350° F. can reduce RF exposure time and improve surface sintering.

There is also indication that calculated porosity of the material can be controlled by mold pressure, sensitizer level, exposure time and polymer particle size. Porosity appears to reach a maximum just before polymer starts to melt and mechanical strength also appears to peak near this same point. This point is observed when the current just starts to increase from a minimum after first having passed a maximum.

It is also seen that organic additives (Run 10 Table IVb), even in small amounts, seem to interfere with the sintering though Runs 7 and 8 of Table IVb show an exception to this effect.

It was also found that the RF sintering of polycarbonate or poly(phenylene sulfide) polymer powders to form porous materials was not successful because the samples melted. It is believed that the sharp melting points of these polymers was responsible for this behavior.

EXAMPLE V

The effect of preheating conditions on the RF sensitivity of various inorganic materials was examined. These tests were conducted in the apparatus previously described in Example I. These tests measured weight (moisture) loss as well as RF responsiveness of the inorganic material after the preheating treatment. The results are presented in Table V below. In each test about 18 g of sample was employed for a 2 minute preheat at the indicated temperature.

                  TABLE V                                                          ______________________________________                                         Run     Inorganic Moisture    Time sec/to                                      No.     Material.sup.(a)                                                                         Loss, Wt %  Reach Temp °F.                            ______________________________________                                         1       A         7.1         310/600°+                                 2       B         1.9         240/250°.sup.(b)                          3       C         9.6         150/Glow.sup.(b)                                 4       D         12.2        28/Glow.sup.(b)                                  5       E         13.9        70/Glow.sup.(b)                                  6       F         0           50/Glow.sup.(b)                                  7       G         11.         7/450°                                                                  400° C. Preheat                           8       A         9.1         360/400°                                  9       B         31.8        240/400°                                  10      C         11.8        130/Glow.sup.(b)                                 11      D         21.1        40/Glow.sup.(b)                                  12      E         15.6        67/Glow.sup.(b)                                  13      F         0           40/Glow.sup.(b)                                  14      G         20.4        30/Glow.sup.(b)                                                                600° C. Preheat                           15      A         9.9         360/270°                                  16      B         --          --                                               17      C         16.8        500/Glow.sup.(b)                                 18      D         22.2        40/Glow.sup.(b)                                  19      E         16.3        58/Glow.sup.(b)                                  20      F         0           30/Glow.sup.(b)                                  21      G         21.8        60/Glow.sup.(b)                                  ______________________________________                                          .sup.(a) A is a precipitated hydrated amorphous silica, Hi Sil 233.            B is a hydrated alumina, Hydral 705.                                           C is a Western bentonite clay, Nygel.                                          D is a crystalline sodium aluminum silicate (zeolite), Arogen 3001.            E is an amorphous sodium aluminum silicate, Zeolex 7 UD.                       F is an American process zinc oxide, Type B, lead free.                        G is a crystalline sodium aluminum silicate, Zeolite ZLD 1000.                 .sup.(b) Material glowed on exposure to RF energy.                       

It can be seen from Table V that RF responsiveness actually increased for materials B, E and F after preheating at 200°, 400° and 600° C. while material A decreased in RF responsiveness as the preheat temperature increased, apparently due to increasing moisture loss. Material C and G first showed an increase in RF responsiveness on increasing preheat temperature from 200° to 400° C. but then decreased in RF responsiveness as the temperature was raised from 400° to 600° C. This decrease may have been due to a collapse of crystalline structure in these materials at about 500° C. Material D appeared relatively insensitive to preheat temperature in terms of RF responsiveness even though significant moisture loss occurred at each temperature which indicates that the RF responsiveness of this material was not significantly dependent on its water content.

EXAMPLE VI

Tests were conducted using the RF heating apparatus described in Example I at 1.5 inch electrode distance to examine the effect of polymer particle size on RF responsiveness of polymer particles (20 g) admixed (ballmilled 30 minutes) with 5 php of crystalline sodium aluminum silicate, Arogen 3001, Type 4A zeolite, particle size>1 micron, surface area>200 m² /g (BET/method) from J. M. Huber Corp. (The zeolite was not preheated.) Exposure time in each run was 45 seconds and the recorded temperature was obtained by use of a needle pyrometer. The results obtained in these runs are presented below in Table VI.

                  TABLE VI                                                         ______________________________________                                         Run                                                                            No.      Particle size (mesh)                                                                           Δ mAmp                                                                            °F.                                   ______________________________________                                         A. Poly(phenylene Sulfide).sup.(a)                                             1        >12             18-35    360                                          2        12 < 20         20-36    395                                          3        20 < 40         20-34    345                                          4        40 < 50         20-32    315                                          5        <50             19-29    283                                          6        Unfractionated  20-33    340                                          B. High MW HD Polyethylene.sup.(b)                                             7        >20             17-32    220                                          8        20 < 40         17-32    210                                          9        40 < 50         16-30    192                                          10       <50             16-30    189                                          11       Unfractionated  18-31    200                                          C. Ultrahigh MW Polyethylene.sup.(c)                                           12       >50             17-37    267                                          13       <50             18-36    264                                          ______________________________________                                          .sup.(a) Ryton ® MR-03                                                     .sup.(b) Marlex ® HXM 50100                                                .sup.(c) Hercules 1900 IV 22                                             

As seen in Table VI above the layer polymer particles appear to be more responsive to RF heating. This may be due to a greater concentration of RF sensitizer on the surface of said particles causing greater energy absorption on the surface of the larger particles. If surface overheating is expected to be very detrimental then it will be preferable to use a smaller particle size for the polymer and with a narrow particle size distribution if possible. If coarser polymer particles have to be used it would be preferable that they be mixed with smaller polymer particles if possible.

EXAMPLE VII

Other tests were conducted with crystalline or amorphous sodium aluminum silicates of various particle sizes used as RF sensitizers in ultrahigh molecular weight polyethylene (Hercules UHMWPE 1900, IV 22 powder). All of the sodium aluminum silicates were supplied by J. M. Huber Corp. The tests were made using the RF equipment previously described in Example I.

In each test 50 g of UHMWPE powder was ball milled for 30 min. with 2.5 g of the sodium aluminum silicate. A 20 g portion of the blend was placed in the sample holder cavity taking care that the bed thickness was uniform. The lid was placed on top of the sample holder and the material exposed to RF cross-field at an electrode distance of 1.5 inch for 30 and 60 seconds. As before, the temperature was measured with a needle pyrometer. The ΔmAmp value is the difference between the actual mAmp reading with sample in place and the mAmp reading for RF exposure of the empty sample holder. The sample holder was cooled to room temperature before each test was run.

The results obtained in these tests are presented in Table VII below.

                                      TABLE VII                                    __________________________________________________________________________     Example VII                                                                                   BET        RF Exposure Time                                     Run                                                                               NaAl Ave. Particle                                                                         Surface                                                                              Wt. loss                                                                            30 sec. 60 sec.                                      No.                                                                               Silicate.sup.(a)                                                                    Size, microns                                                                         Area, m.sup.2 /g                                                                     %    ΔmAmp                                                                         °F.                                                                        ΔmAmp                                                                         °F.                              __________________________________________________________________________     1  A-1  5-7    180-225                                                                              6.sup.(b)                                                                           17-37                                                                               241                                                                               17-37                                                                               283                                     2  A-2  5-7    100-130                                                                              8.sup.(b)                                                                           11-31                                                                               213                                                                               11-31                                                                               273                                     3  A-3  5-7    60-80 3.5-5.5.sup.(b)                                                                      8-15                                                                               140                                                                                8-15                                                                               165                                     4  A-4  5-7    30-40 7.sup.(b)                                                                            5-12                                                                               121                                                                                5-12                                                                               139                                     5  A-5  7.6    23    15.4.sup.(c)                                                                        3-8  101                                                                               3-8  111                                     6  A-6  4.9    23    15.4.sup.(c)                                                                         3-13                                                                               109                                                                                3-13                                                                               119                                     7  C-7  3.7    3     20.4.sup.(c)                                                                         7-15                                                                               139                                                                                7-16                                                                               196                                     8  C-8  0.78   200   23.2.sup.(c)                                                                        15-39                                                                               243                                                                               14-36                                                                               270                                     9  C-9  0.91   200   23.4.sup.(c)                                                                        15-38                                                                               237                                                                               14-36                                                                               263                                     10  C-10                                                                               2.2    519   24.9.sup.(c)                                                                        12-17                                                                               163                                                                               11-16                                                                               217                                     11  C-11                                                                               0.99   415   26.7.sup.(c)                                                                        25-56                                                                               273                                                                               25-57                                                                               415                                     12  C-12                                                                               0.78   218   23.3.sup.(c)                                                                        15-41                                                                               250                                                                               15-40                                                                               278                                     13  C-13                                                                               0.83   274   23.5.sup.(c)                                                                        18-47                                                                               260                                                                               18-47                                                                               295                                     __________________________________________________________________________      .sup.(a) Samples designated A are amorphous while those showing C are          crystalline.                                                                   .sup.(b) Moisture loss.                                                        .sup.(c) Loss on ignition.                                               

The results shown in Table VII demonstrate that for the amorphous or crystalline sodium aluminum silicates a significant improvement in RF sensitizing efficiency is seen in using those materials which have high surface area and small particle size. It is also expected that the RF sensitizer materials of smaller particle size will have less effect on polymer physical properties than similar materials of larger particle size. 

I claim:
 1. A method for enhancing the radio frequency sensitivity of a substance suitable for radio frequency enhancement comprising admixing with said substance suitable for radio frequency enhancement a radio frequency sensitizing amount of an inorganic radio frequency sensitizer treated essentially to remove absorbed water and/or other volatiles said sensitizer chosen from the group consisting of (1) zinc oxide (American Process) and (2) bentonite clay.
 2. A method of claim 1 wherein said inorganic radio frequency sensitizer is bentonite clay.
 3. A method of claim 2 wherein said substance suitable for enhancement of radio frequency sensitivity is a polymer.
 4. A method of claim 3 wherein said polymer is ultra high molecular weight polyethylene.
 5. A method of claim 4 wherein said sensitizer has been heat treated.
 6. A method of claim 5 wherein said sensitizer is admixed in an amount in a range of about 1 weight percent to about 20 weight percent based on total composition. 